Fever is a commonly used medical indicator characterized by elevation of body temperature above a normal range. The normal range for human body temperature may vary, from person to person, based on external temperatures, age, sex, and level of activity of the person, and time of the day. For example, most individuals tend to have lower temperature readings in the second half of the sleep cycle. Further, body temperature measurements tend to change when a person is cold, sleepy, or hungry.
The average oral temperature is an adult is about 36.8° C. (98.2° F.). However, individuals are not expected to have the same exact temperature at all times during the course of a day. Temperature cycles regularly vary throughout the day and are believed to be controlled by a person's circadian rhythm/cycle.
Body temperatures are not always constant and may vary during the day based on the circadian cycle. As such, normal body temperature may vary as much as 1° F. (0.5° C.) for individuals throughout a day, which can lead to errors in assessing elevation of body temperature as true fevers indicating physiological disease processes.
For individuals, throughout the day, because body temperature varies with circadian cycle, it is known that two different temperature thresholds can be used to determine fever depending on time of the day.